1. Field of the Invention
The present invention relates to a differential assembly, and more particularly to a locking differential assembly for motor vehicles, having an electro-mechanically externally actuated friction clutch assembly.
2. Description of the Prior Art
Conventionally, differentials well known in the prior art, are arranged in a power transmission system of a motor vehicle to allow a pair of output shafts operatively coupled to an input shaft to rotate at different speeds, thereby allowing the wheel associated with each output shaft to maintain traction with the road while the vehicle is turning. Conventional differentials include a differential case defining a gear chamber, and disposed therein, a differential gear set including at least one input pinion gear, and a pair of output side gears non-rotatably coupled to corresponding axle shafts. Such a device essentially distributes the torque provided by the input shaft between the output shafts. However, this type of differentials known in the art as an open differentials, i.e. a differential where movements of its various internal components are not restricted in any significant fashion, is unsuitable in slippery conditions where one wheel experiences a much lower coefficient of friction than the other wheel; for instance, when one wheel of a vehicle is located on a patch of ice or mud and the other wheel is on dry pavement. In such a condition, the wheel experiencing the lower coefficient of friction loses traction and a small amount of torque to that wheel will cause a xe2x80x9cspin outxe2x80x9d of that wheel. Since the maximum amount of torque, which can be developed on the wheel with traction, is equal to torque on the wheel without traction, i.e. the slipping wheel, the engine is unable to develop any torque and the wheel with traction is unable to rotate. Thus, the necessity for a differential which limits the differential rotation between the output shafts to provide traction on slippery surfaces is well known.
Prior methods of limiting slippage between the side gears and the differential case usually employ a frictional clutch mechanism typically disposed between at least one of the side gears and an adjacent surface of the gear case, such that the clutch pack is operable to limit relative rotation between the gear case and the one side gear. A number of methods have been developed to limit wheel slippage under such conditions.
Limited slip differentials conventionally use the frictional clutch mechanism and a bias mechanism, usually a spring, to apply an initial preload between the frictional clutch mechanism and the differential casing. However, such preloaded clutches are usually always engaged, and thus are susceptible to wear, causing undesirable repair and replacement costs. Also, such a preloaded clutch mechanism may lock the output shafts together in situations where differential rotation is necessary.
Another method of limiting slippage involves the use of a selectively controllable frictional clutch mechanism as a differential locking device actuated manually by a vehicle operator or by a signal from an appropriate electronic control unit, to restrict the movements of the internal components of the differential. The frictional clutch mechanism may be actuated by various hydraulic or electromagnetic mechanisms, which conventionally constructed of elements disposed inside the differential casing. However, the locking differentials occupy bigger space and are often quite complex, cumbersome and expansive to manufacture.
Thus, there is a need for a lockable differential that is simple, compact and inexpensive to manufacture.
The present invention provides an improved electronically controlled locking differential assembly having an externally actuated friction clutch assembly.
The differential assembly in accordance with the preferred embodiment of the present invention includes a rotatable differential case forming a housing, a differential gearing rotatably supported in the case, and a pair of opposite side gears in meshing engagement with the differential gearing to permit differential rotation thereof. The differential assembly includes a friction disk clutch assembly disposed within the differential case and provided to lock the differential assembly. The friction clutch assembly includes a number of alternating outer friction plates non-rotatably coupled to the differential case and inner friction plates splined to one of the side gears. An annular actuator plate is arranged within the differential case between the friction clutch assembly and an adjacent surface of the differential case, and is adapted to axially reciprocate within the case for loading the friction clutch plates. An electronic selectively controllable actuator assembly is provided for axially displacing the actuator plate in order to load the friction assembly when needed, thus providing the differential assembly with a locking function. The actuator assembly comprises an electric motor mounted to an axle housing and provided for rotating a drive screw via a gear reducer. In turn, the drive screw axially drives a clutch actuating sleeve through a complimentary threaded pusher block. The clutch actuating sleeve is provided coaxially to the side gear to apply an axial force to the actuator plate in order to compress and, thus, actuate the friction clutch assembly.
Therefore, the locking differential assembly in accordance with the present invention provides a simple, compact and inexpensive locking differential assembly.